Azathioprine is well known for its immunosuppressive properties in the treatment of recalcitrant Inflammatory Bowel Disease (IBD). It has been shown to eliminate the need for corticosteroids in about 75% of patients with Crohn's disease with a median response time of 16 weeks. Although azathioprine has proven clinical efficacy, physicians have been reluctant to use this immune modifying agent in the treatment of severe active disease on account of the delayed clinical response times. The delayed clinical response observed in patients on azathioprine therapy may be due in part to the drug's poor oral bioavailability. Indeed, the bioavailability of oral azathioprine ranges from 5-037%, and may explain why steady-state drug levels can only be obtained after 2 months of chronic therapy. A recent pilot study from the Mayo clinic has achieved improved clinical response times in their patients with severe active Crohn's disease with their use of intravenous therapy. This study suggests that intravenous therapy may improve clinical response time through improved drug bioavailability. This study will compare high dose intravenous azathioprine followed by oral azathioprine with conventional oral azathioprine alone in the treatment of Crohn's disease.